可指人或物；在定语从句中作主语，宾语，表语。指人时，相当于who或 whom；指物时，相当于which。一般不用于非限制性定语从句; 不可置于介词后作宾语。如：
（1）A letter that/which is written in pencil is difficult to read. (主语)
Do you know the gentleman that/who spoke just now?
（2） You can take anything ( that) you like. （宾语）
What is the question (that/which) they are talking about?
Here is the man ( who/whom/that ) you want to see.
Our hometown is no longer the one (that) it used to be.
我们的家乡已不再是过去的那个了。（= Our hometown is not the same as it used to be.= Our hometown is different from what it used to be。= Our hometown is not what it used to be.）
（1）The book which/that was on the desk was bought by my father.（主语）
（2）The book (which/that) I bought yesterday is very interesting.（宾语）
1. They have started an investigation, ______ are being kept secret.
A. the details of which B. of which the details
C. whose the details D. the details of it
2. Sailing across the ocean alone was an achievement ______ took courage.
A. what B. who C. which D. where
3. A survey was conducted in Shanghai _______
interviewees were asked if they wanted to be a factory worker．
A. when B. whether C. that D. where
4. The city was attacked by______ terrible weather______
few citizens had ever experienced before．
A. so, as B. so, that C. such, that D. such, as
5.In our city there are several big public parks ______ many people can go to
enjoy the beautiful scenery.
A. in which B. at which C. from which D. to which